1. Field of the Invention
The present invention relates to a reticle defect inspection apparatus and a reticle defect inspection method. The present invention relates particularly to a calibration for an offset and gain of sensor amplifier means that normalizes a TDI sensor output.
2. Background Art
In order to form patterns on a substrate (called also “wafer”) in a semiconductor manufacturing process, the patterns are exposure-transferred onto the wafer by a scale-down projection exposure apparatus so-called “stepper” using an original drawing pattern (called also “reticle or mask” and hereinafter generically called “reticle”) formed with circuit patterns. The reticle has patterns are normally formed on a light-transmitted glass substrate by a light-shielding material. Chromium (Cr) has been widely used as the light-shielding material. As a phase shift mask corresponding to one super-resolution technique, a halftone type phase shift mask in which a semi-transparent film comprised of molybdenum silicide (MoSi) or the like is formed as a light-shielding film, and a Levenson type phase shift mask to which a phase shift effect is imparted by changing the thickness of a glass substrate, are trying to be used. A pattern writing apparatus using an electron beam, capable of writing each micro circuit pattern is used in the manufacture of such a mask. When a pattern defect exists in the reticle, the defect is transferred onto the wafer. It is therefore necessary to perform a reticle defect inspection.
As reticle defect inspection methods, there are known a Die-to-Die inspection method and a Die-to-Database inspection method. The Die-to-Die inspection method is a method for comparing optical images of the same pattern located in different positions with each other. On the other hand, the Die-to-Database inspection method is a method for comparing a reference image generated from writing data (CAD data) used upon reticle fabrication and an optical image of a pattern of an actual reticle.
In order to generate the optical image, a charge storage type TDI (Time Delay Integration) sensor and a sensor amplifier for amplifying the output of the TDI sensor are used (refer to, for example, Japanese Patent Application Laid-Open No. 2004-271444). Since the contrast between the light-shielding film and the glass substrate is obtained to some extent in the halftone type phase shift mask in the case of an inspection by transmitted light, a technique to recognize a mask pattern with a light intensity signal of a sensor image light-received by a detection optical system in a manner similar to a chromium mask thereby to perform a defect decision can be adopted. There is a case in which the utilization of reflected light on a mask surface makes it easy to obtain contrast depending on the shape of a defect. There is also known an inspection apparatus equipped with a reflection inspection optical system in applications such as a particle inspecting function, etc.
It is known that a calibration for the offset and gain of the sensor amplifier is performed prior to the above comparison between the reference image and the optical image (refer to, for example, Japanese Patent No. 3410847). According to the Japanese Patent No. 3410847, a black region (light-shielding film region or halftone film region) having an area sufficiently broader than an imaging area of a TDI sensor for imaging transmitted light is imaged upon inspection by the transmitted light to calibrate the offset. Thereafter, a white region (glass substrate) having an area sufficiently wider than the imaging area of the TDI sensor is imaged to calibrate the gain. Upon inspection by reflected light, each part in which a chromium or halftone film exists becomes a white region by its reflection, and part of the glass substrate becomes a black region because no reflected light exists therein.
However, miniaturization of each pattern written on a product reticle has been advanced in recent years, and a black region or a white region having a sufficiently wide area might not exist in a product pattern itself. It has also been partly practised to prepare a black or white region having a sufficiently wide area other than each product pattern for calibration. However, the occupied area of the product pattern has been enlarged, and the pattern for the calibration becomes a matter of being not necessarily prepared. In this case, an offset/gain calibration reticle (hereinafter called “calibration reticle”) in which black and white regions of the same kind of film as the product reticle are formed has been used. After the offset and gain of the sensor amplifier have been calibrated using the calibration reticle, it is replaced with the corresponding product reticle to execute a defect inspection. Such a reticle replacement incurs a reduction in throughput and causes a risk that errors will be contained in the amplitude of a sensor signal and the offset due to an error in finished accuracy of a light-shielding film between the calibration reticle and the product reticle.
When the thickness of the glass substrate differs, transmittance of light varies. Therefore, there is a possibility that the offset and gain of the senor amplifier both optimally adjusted or calibrated using the calibration reticle will not be rendered optimal in the product reticle.
Due to the reasons mentioned above, it is desirable to calibrate the offset and gain of the sensor amplifier using the product reticle itself corresponding to the reticle to be inspected, without using the calibration reticle.